This invention claims priority of a German filed patent application DE-A-199 48 797.9.
This invention concerns a substrate holder and the use of the substrate holder according to the present invention in a highly accurate measuring instrument. The invention concerns in particular a substrate holder which is configured in such a way that it is suitable for a defined support of a substrate and thereby contributes to enhancing the measurement accuracy of the highly accurate measuring instrument.
U.S. Pat. No. 5,671,054 discloses a method and an apparatus for determining the position of patterns on substrates that have a specific thickness. The substrate is placed onto a measurement stage that, in order to support the substrate, has three defined point-like support elements. The thickness of the substrate is determined using a reference substrate that is first measured. Then the substrate provided for measurement is measured. A data processing element then calculates the corresponding thickness differences from the differences between the reference substrate and the substrate to be measured, and incorporates them into the calculation for further determination of the position of the pattern. This arrangement has a considerable disadvantage, however, in that it is not very flexible, since only fixed mask sizes can be used or measured in this case. It is moreover problematic that if masks with different mask thicknesses need to be measured, the focal plane changes and is no longer located exactly on the surface of the substrate.
A substrate holder is also known from the article xe2x80x9cAdvanced Mask Metrology System for Up to 4 Gbit DRAM,xe2x80x9d published in SPIE, Vol. 3096-0277-786X/97, pages 433-444. This presents a universal substrate holder that has the support points or support elements arranged correspondingly on the substrate holder surface in accordance with the various sizes of the substrate types used. The support elements are configured such that they differ in height. For example, the support elements for the substrates with smaller dimensions are not as high as the support elements for substrates with larger dimensions. A disadvantage of this substrate holder is that it is not suitable for transmitted light.
It is therefore the object of the present invention to create a substrate holder that is suitable for measurement with transmitted light and makes available a defined support for the substrates.
According to the present invention this object is achieved by a substrate holder of a substrate that comprises
a) a one-piece frame having a flat upper surface;
b) an opening having a peripheral rim that is configured in the substrate holder; and
c) three support elements that are shaped on the peripheral rim of the opening, on which are mounted spheres on which the substrate rests; and the spacing from the upper surface of the spheres to the flat upper surface of the substrate holder corresponds to the standard thickness of the substrate type being used.
A further object of the invention is to use the substrate holder in a highly accurate measurement system with which substrates can be measured in transmitted light; and that the substrate holders used for the purpose make available a defined support for the substrates and for determination of the deviation of a substrate thickness from a predefined standard thickness.
This object is achieved in that the substrate holder is suitable for determination of the deviation from standard thickness in the case of substrates of one type, and is used in a highly accurate measuring instrument.
The substrate holder according to the present invention and the use according to the present invention of the substrate holder make possible measurement of the substrate in both reflected and transmitted light. The substrate holders are configured in such a way that all the surfaces of the substrates to be measured lie at an identical height. This height is defined in such a way that the surfaces of the substrates lie in the Abbxc3xa9 plane, which minimizes Abbxc3xa9 errors. The substrate holders are configured as frames in which is defined an opening that is delimited by a peripheral rim. Support elements, on which point supports are configured, are provided on the peripheral rim of the opening. In a preferred exemplary embodiment, three support points are distributed correspondingly along the peripheral rim of the opening. The three-point support results in an exact calculation of the mask deflection, which makes possible exact calculation and correction of the position data. The substrate holders are configured such that a different substrate holder is used for each substrate type. The outside dimensions of each substrate holder are the same. The difference arises from the fact that in the substrate holders, the defined openings are correspondingly based on the size of the substrate type used. The support elements and the point supports thereby made available arc arranged on the peripheral rim of the openings in such a way that the flat upper surface of the substrate holder aligns with the surface of the substrate. Small deviations from the standard thickness of a substrate type can be corrected by focusing with the optical system of the measuring instrument first on the surface of the substrate and then on the flat upper surface of the substrate holder. From the difference in focal positions it is then possible to ascertain a thickness deviation of the substrate, which is then incorporated into the exact calculation of the mask deflection. A code that allows individual processing of the substrate holder by the measurement and correction software can also be mounted on the substrate holder. Long-term monitoring (drift monitoring and optionally also drift correction) is made possible by integrated reference points. In addition, the substrate holders can be configured such that the total weight of the substrate holder and substrate placed thereon is always approximately the same for the various substrate types. Distortions and deformations at the X/Y carriage and at the mirror body can thereby be kept constant to a first approximation. For this purpose, corresponding programs which correct these constant external influences are provided in a calculation unit of the measuring instrument.